Dual shell dental appliance and material constructions

ABSTRACT

Improved dental appliances and polymeric sheet compositions are disclosed. The polymeric sheet compositions are useful for making dental appliances having outer layers comprised of a material having a modulus of from about 1,000 MPA to 2,500 MPA (“hard”) and an inner core comprised of elastomeric material or materials having a modulus of from about 50 MPa to 500 MPa (“soft”), which exhibit improved flexibility and strength, and better stain and tear resistance than currently available materials and dental appliances.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part application of U.S.application Ser. No. 16/211,176, filed Dec. 5, 2018, which is acontinuation-in-part application of International Application Serial No.PCT/US2018/035384, which has International filing date May 31, 2018, andfurther claims the benefit of U.S. Provisional Application No.62/512,786, filed May 31, 2017 and of U.S. Provisional Application No.62/590,627, filed Nov. 26, 2017, each of which are incorporated hereinby reference in their entirety.

TECHNICAL FIELD

Compositions in the form of polymeric sheets are disclosed. Thepolymeric sheets are useful, for example, in a dental appliance, and areconstructed of layers that impart flexibility and strength and stainresistance to devices made from the sheets.

BACKGROUND

There is a need for improved orthodontic and dental appliances capableof facilitating orthodontic tooth movements, stabilizing tooth positionsor protecting teeth from potentially damaging outside forces. Existingmaterials and products are constructed from single layer materials,bi-layer materials or tri-layer materials which have limitedfunctionality and may suffer from performance deficiencies. Aligners areplastic shells which fit over teeth designed to apply translational orrotational forces to teeth. Their ability to accurately move teeth islimited by their effective modulus, elasticity and ability to resistcreep and stress relaxation. Additionally, they generally should beresistant to staining and environmental stress cracking.

Appliances for protection of teeth, for example, sports mouth guards,and dental splints have contradictory requirements. On the one hand,they should be capable of dissipating impact forces and on the otherhand should be thin and not interfere with the natural occlusion of aperson's teeth or impede speaking.

BRIEF SUMMARY

In one aspect, a composition comprised of at least two outer layers Aand C and a middle layer B is provided. The A and C layers individuallycomprise a thermoplastic polymer having a modulus of from about 1,000MPa to 2,500 MPa and a glass transition temperature and/or melting pointof from about 80° C. to 180° C. and the middle B layer is comprised ofat least an elastomer having a modulus of from about 50 MPa to about 500MPa and one or more of a glass transition temperature and/or meltingpoint of from about 90° C. to about 220° C.

In one embodiment, the A and C layers are comprised of one of more of aco-polyester, a polycarbonate, a polyester polycarbonate blend, apolyurethane, a polyamide or a polyolefin.

In another embodiment, the middle B layer is comprised of one or more ofa polyurethane elastomer, a polyolefin elastomer, a polyester elastomer,a styrenic elastomer, a polyamide elastomer, a cyclic olefin elastomer,an acrylic elastomer, an aromatic or aliphatic polyether and a polyesterpolyurethane.

In yet another embodiment, the middle B layer material has a compressionset of less than 35%, 30%, 25%, 20% or 10% after 22 hours at 25° C.

In still another embodiment, the A and C layers have a lateral restoringforce of less than 100 N (Newtons) per cm², 50 N per cm², 25 N per cm²,or 10 N per cm², when displaced by 0.05 mm to 0.1 mm relative to eachother.

In another embodiment, the interlayer peel strength between the A and Clayers and the B layer is greater than 50 N per 2.5 cm.

In one embodiment, the combined thickness of the A, B and C layers isfrom about 250 microns to about 2,000 microns and the combined thicknessof the A and C layers is from 25 microns to 750 microns, from 50 micronsto 1000 microns, from 100 microns to 700 microns, from 150 microns to650 microns, from 100 microns to 200 microns, from 200 microns to 600microns, 100 microns, 125 microns, 150 microns, 175 microns, 200microns, 225 microns, 250 microns, 275 microns, or 300 microns.

In still other embodiments, one or more of the A and C layers comprise amicrocrystalline polyamide comprised of from 50 to 100 mole % of C6 toC14 aliphatic diacid moieties, and about 50 to 100 mole % of4,4′-methylene-bis(cyclohexylamine) (CAS [1761-71-3]), having a glasstransition of between about 100° C. and 180° C., a heat of fusion ofless than 20 J/g and a light transmission of greater than 80%.

In another embodiment, one or more of the A and C layers comprises aco-polyester comprised of: (a) a dicarboxylic acid component comprising70 mole % to 100 mole % of terephthalic acid residues, and (b) a diolcomponent comprising i) 0 to 95% ethylene glycol, ii) 5 mole % to 50mole % of 2,2,4,4-tetramethyl-1,3-cyclobutanediol residues, and iii) 50mole % to 95 mole % 1,4-cyclohexanedimethanol residues, iiii) 0 to 1% ofa polyol having three or more hydroxyl groups, wherein the sum of themole % of diol residues i) and ii) and iii) and iiii) amounts to 100mole % and the copolyester exhibits a glass transition temperature Tgfrom 80° C. to 150° C.

In another embodiment, the middle B layer comprises an aromaticpolyether polyurethane having a Shore hardness of from about A90 to D55,from about A85 to D60, or from about A80 to D65, and a compression setof less than 35%, wherein the interlayer peel strength between the A andC layers and the B layer is greater than 50 N per 2.5 cm.

In one embodiment, one or more of the A and C layers comprises apolyurethane comprised of (a) a di-isocyanate comprising 80 mole % to100 mole % of methylene diphenyl diisocyanate residues and/orhydrogenated methylene diphenyl diisocyanate and (b) a diol componentcomprising i) 0 to 100 mole % hexamethylene diol and ii) 0 to 50 mole %1,4-cyclohexanedimethanol, wherein the sum i) and ii) amounts to greaterthan 90 mole % and the polyurethane has a glass transition temperatureTg from about 85° C. to about 150° C.

In another aspect, a dental appliance conformal to one or more teethmade from a composition or a polymeric sheet as described herein.

In one embodiment of the dental appliance the combined thickness of theA, B and C layers is from about 250 microns to about 2,000 microns andthe combined thickness of the A and C layers is from 25 microns to 750microns, from 50 microns to 1000 microns, from 100 microns to 700microns, from 150 microns to 650 microns, from 100 microns to 200microns, or from 200 microns to about 600 microns.

In another aspect, a reversibly deformable dental appliance comprised ofa composition or a polymeric sheet material as described herein isprovided, wherein the elastomeric middle layer and the outer layers canreversibly move relative to each other and have a lateral restoringforce of less than 100 N per cm², 50 N per cm², 25 N per cm², or 10 Nper cm² when displaced by 0.05 mm to 0.1 mm relative to each other.

In one embodiment, the elastomeric middle layer comprises a polyurethanehaving a hardness from about A 80 to D 75, A 85 to D 65, or A 90 to D55, e.g., A95, A90, A85, A80, A75, D50, D55, D60, D65 or D70.

In another aspect, a composition, polymeric sheet or dental appliancehaving environmental stress resistance comprised of at least two outerlayers and an elastomeric inner layer, wherein one or more of the outerlayers is a polyester or co-polyester having a modulus of from about1,000 MPa to 2,500 MPa, and the inner layer comprises an elastomerhaving a modulus of from about 50 MPa to about 500 MPa, wherein theinter layer peel strength between at least one outer layer and theelastomer is greater than about 50 N/inch, is provided.

In another aspect, a reversibly deformable dental appliance is provided,wherein the thickness of the outer A layer is from about 175 microns toabout 250 microns, about 100 microns to about 200 microns, e.g., 100microns, 125 microns, 150 microns, 175 microns, 200 microns, 225 micronsor 250 microns, the thickness of the outer C layer is from about 175microns to about 250 microns, 100 microns to 200 microns, e.g., 100microns, 125 microns, 150 microns, 175 microns, 200 microns, 225 micronsor 250 microns, and the thickness of the middle B layer is from 300 to500 microns, wherein the combined thickness of the A, B and C layersfrom 850 to 1,000 microns or from 600 to 800 microns.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic depiction of a cross sectional view of athree-layer sheet with a simple ABC construction. Layer A and layer Cmay be the same or different materials and each layer may be comprisedof one or more materials, or blends or alloys. Layer B may be a singlematerial, a blend of materials or alloys.

FIG. 1B is a schematic depiction of a cross sectional view of a multiplelayer sheet. Each layer A, B and C, may be a comprised of a single layeror multiple layers and each layer may be comprised of one or morematerials or a blend of materials. Layer A may be comprised of more thanone layer, for example, layer a and a′, layer B may be comprised of morethan one layer, for example, layer b and b′, and layer C may becomprised of more than one layer, for example, layer c and c′, asexemplified in FIG. 1B.

FIGS. 2A and 2B are schematic depictions of exemplary test samples fordetermining displacement (FIG. 2A) and lateral restoring force(translational movement; FIG. 2B), of a simple 3-layer sheet comprisedof two rigid outer layers and an inner elastomeric layer, where A, B andC are individual layers of the sheet. In this example, layers A and Care reversibly translated relative to each other and layer B provides arestoring force. In one more specific example, the A B C layers may eachbe about 250 microns thick, and layers A, B and C may be comprised orone or more materials and may each individually comprise one or morelayers.

FIG. 3A is a graphic depiction of displacement/force curves forelastomers having different degrees of hardness. The graph demonstratesthe restoring force (N) generated from translational movement of Layer Arelative to Layer C, having a middle B layer with different hardness TPUelastomers, and that the hardness of the elastomer impacts displacementand restoring force. A harder thermoplastic urethane (TPU) will generatea greater restoring force, but may limit the amount of movement.

FIG. 3B is a graphic depiction of the restoring force (N) as a functionof time (0 to 48 hours) for a given displacement between an A layer anda C layer having a middle B layer with different hardness TPU elastomersin the B layer. TPU 75A has a low compression set and shows the leastinitial force, but the force decays very little over time. TPU 75D has ahigh compression set, and while it shows a much higher initial restoringforce, the force decays rapidly over time.

FIG. 4 is a graphic depiction of the retained force at 5% stress fordifferent constructions exposed to 37° C. and water over a 48-hour timeperiod.

It should be appreciated that the constructions and propertiesillustrated in FIGS. 1-4 are specific examples and not intended to limitthe scope of constructions and testing that may be used. Othermaterials, constructions and sequences of steps may also be performedaccording to alternative embodiments. For example, alternativeembodiments may contain additional layers including tie layers,pigments, optical additives or reinforcing agents and may be constructedin any manner known in the art such as flat sheet extrusion, coextrusionblown film, calendaring, laminating and adhesive bonding. The structures(or polymer sheets) and devices may in some embodiments be made by 3Dprinting or dip coating. One of ordinary skill in the art wouldrecognize and appreciate many variations, modifications, andalternatives of the constructions.

The specification and drawings are, accordingly, to be regarded in anillustrative rather than a restrictive sense. It will, however, beevident that various modifications and changes may be made thereuntowithout departing from the broader spirit and scope of the disclosure asset forth in the claims.

Other variations are within the spirit of the present disclosure. Thus,while the disclosed embodiments are susceptible to various modificationsand alternative constructions, certain illustrated embodiments thereofare shown in the drawings and are described herein. It should beunderstood, however, that there is no intention to limit the disclosureto the specific form or forms disclosed, but on the contrary, theintention is to cover all modifications, alternative constructions andequivalents falling within the spirit and scope of the disclosure, asdefined in the appended claims.

DETAILED DESCRIPTION

Current orthodontic aligners have a very limited elastic range(typically 4% to 7%) and when deformed exhibit a rapid decay of recoveryforce. As a result, it may be necessary to change appliances frequently,increasing manufacturing cost, teeth may not move as desired, and thepatient may experience discomfort from excessively high initial forces.Attempts to improve elastic range by providing a thin outer layer ofelastomer (typically a polyurethane, as described for example in U.S.Pat. No. 9,655,693 B2), can result in a tooth contact surface which isreadily deformed, reducing accuracy of tooth movement, and may increasethe propensity for unsightly staining by common foods, beverages orcigarettes. U.S. Pat. No. 6,524,101 describes dental appliances havingregions with different elastic modulus and appliances having addedstiffening elements. Non-staining polyurethanes used for fabricatingdental appliances such as Zendura® A available from Bay Materials, LLC(Fremont, Calif.), have excellent properties but are hygroscopic,requiring rigorous drying prior to thermoforming, may initially beuncomfortable, are difficult to clean and may not be ideal for someapplications.

Many other polyurethanes must also be dried prior to thermoforming,adding time and cost to the manufacturing process. Aromatic polyestersor co-polyesters may be used to form aligners; however, they exhibitpoor chemical resistance and low impact and tear strength. Alignersconstructed from stiff materials such as polyesters or rigidpolyurethanes have a high modulus, for example greater than about 1,000or 1,500 MPa, and when deformed can exert excessive forces on teethcausing discomfort and potential damage to tooth roots. Highlyelastomeric polymers such as thermoplastic polyurethane elastomers(TPU), styrenic elastomers (such as SBS, SEBS, SIS for example) have lowmodulus (typically less than 100 or 200 MPa), which may be insufficientfor moving teeth and are readily stained making them of limited utilityfor producing aligners.

The present disclosure is based on the discovery that many of thedeficiencies in prior art materials, and dental appliances constructedfrom them can be reduced or eliminated with a sheet or device havingouter layers comprised of a material having a modulus of greater thanabout 1,000 MPA up to 2,500 MPA and an inner elastomeric layer or corecomprised of elastomeric material or materials having a modulus of fromabout 50 MPa to 500 MPa, which can be non-staining, has a lower costthan rigid urethanes, exhibits improved elastic properties, and hassurprisingly greater environmental stress cracking resistance.

A polymeric sheet or device may be comprised of more than two rigidlayers, for example a third rigid layer may be disposed between two ormore elastomeric layers. The multilayer construction provides a dualshell dental appliance which may be adapted to moving teeth, retainingteeth in an existing position, or protecting teeth from impact. Asdisclosed herein, the outer shell material that contacts the teeth maybe substantially rigid to accurately mate with the teeth providingprecise forces while maintaining the ability to exert a more nearlyconstant force over longer distances.

By selecting appropriate outer and inner material modulus and thickness,two or more substantially rigid shells may be reversibly displacedrelative to each other to a greater extent than a rigid material ofcomparable thickness and shape providing a dental appliance that canapply desired forces to teeth with a greater range of movement while notcreating excessive forces or exhibiting excessive stress relaxation whendeformed. While not limiting the disclosure to specific constructions, asheet or dental appliance may be referred to herein as a “dual shell”sheet or appliance. A “dual shell” sheet or appliance may comprise twoor more shells or layers. The shells or layers may have the same or adifferent thickness. A series of dental appliances comprised of this“dual shell” construction may be used to move teeth in incrementalstages wherein two or more appliances may be constructed of the same ordifferent materials. Dental appliances may be constructed bythermoforming a dual shell material over a model of one or more teeth ormay be constructed by sequentially thermoforming rigid and elastomericprecursor sheets or by sequentially dip coating a model with polymersolutions or polymer forming monomers or oligomers which may optionallycured or otherwise post processed. The inventors have discovered thatthis unique construction can significantly reduce the amount of stresscracking that a shell or materials exhibits, thereby expanding the rangeof materials which can be used in the sheets or appliance.

Definitions

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the disclosed embodiments (especially in thecontext of the following claims) are to be construed to cover both thesingular and the plural, unless otherwise indicated herein or clearlycontradicted by context. The terms “comprising,” “having,” “including,”and “containing” are to be construed as open-ended terms (i.e., meaning“including, but not limited to,”) unless otherwise noted. The term“connected” is to be construed as partly or wholly contained within,attached to, or joined together, even if there is something intervening.The phrase “based on” should be understood to be open-ended, and notlimiting in any way, and is intended to be interpreted or otherwise readas “based at least in part on,” where appropriate. Recitation of rangesof values herein are merely intended to serve as a shorthand method ofreferring individually to each separate value falling within the range,unless otherwise indicated herein, and each separate value isincorporated into the specification as if it were individually recitedherein. All methods described herein can be performed in any suitableorder unless otherwise indicated herein or otherwise clearlycontradicted by context. The use of any and all examples, or exemplarylanguage (e.g., “such as”) provided herein, is intended merely to betterilluminate embodiments of the disclosure and does not pose a limitationon the scope of the disclosure unless otherwise claimed. No language inthe specification should be construed as indicating any non-claimedelement as essential to the practice of the disclosure.

The term “dental appliance” is used herein with reference to any deviceplaced in or on the teeth of a subject. Dental appliances include butare not limited to orthodontic, prosthetic, retaining, snoring/airway,cosmetic, therapeutic, protective (e.g., mouth guards) andhabit-modification devices.

The term “ASTM D638”, is used herein with reference to the test forPlastics Tensile Strength.

The term “ASTM D1364”, is used herein with reference to the test forinter layer peel strength.

The term “compression set” is used herein with reference to thepermanent deformation of a material when a force is applied and removed.Unless specified otherwise, compression set is measured according toASTM D 305-B at specified time and temperature, for example 22 hours at23° C.

The term “flexural modulus” is used herein with reference to therigidity of a material and/or resistance of the material to deformationin bending. The higher the flexural modulus of the material, the moreresistant to bending it is. For an isotropic material the elasticmodulus measured in any direction are the same.

The term “hardness” is used herein with reference to a Shore hardnessscale, and unless otherwise stated is measured according to ASTM D 2240.A durometer measures the penetration of a metal foot or pin into thesurface of a material. There are different durometer scales, but Shore Aand Shore D are commonly used. Materials with higher durometer valueswill be harder compared to materials with a lower durometer value. Shorehardness and modulus are generally correlated and can be converted byapproximation if only one value is known by methods described in theart.

The expressions “modulus,” “Young's modulus” and “elastic modulus” areused herein with reference to the rigidity of a material and/orresistance of the material to stretching. The higher the modulus of thematerial, the more rigid. The flexural modulus and elastic modulus of amaterial may be the same or different. For isotropic materials such asA, B and C, flexural modulus and modulus (which may also be referred toas elastic modulus) are substantially the same and one or the other maybe measured dependent upon the circumstances. For polymers, themechanical properties including elastic modulus and other properties maybe measured as proscribed by ASTM D 638. Flexural modulus may bemeasured by the test listed in ASTM D790), and uses units of force perarea. Unless designated otherwise, “modulus” refers to elastic modulus.

The term “polymeric sheet” is used interchangeably herein with the term“plastic sheet”.

The term “lateral restoring force” with respect to A and C layers of apolymeric sheet is used with reference to the force which may be exertedby one layer which has been translated relative to another layer whichis fixed in position. If the A and C layers are caused to moveindependently of each other they will subsequently return to theiroriginal positions if not restrained.

“Translational force” refers to the amount of force required to displacean A and C layer from their neutral position a given distance and ismeasured as Newtons per cm2 (N/cm2) at a given displacement where thearea (cm²) is calculated as the overlap area of the A and C layers. Themeasurement can be made by preparing a test sample of known overlap anddisplacing the A and C layers relative to each other a given distance,for example, by applying a force of 0.04 MPa/min using a mechanicalforce tester such as an Instron Materials Tester. The force measured atdifferent displacements is recorded. The lateral translational force andthe lateral restoring force will be the same for an elastic material.

FIG. 2A illustrates the test for determining displacement force. FIG. 2Billustrates the test for determining restoring force. The schematic ofFIGS. 2A and 2B show a simple 3-layer sheet comprised of two rigid outerlayers and an inner elastomeric layer, where layers A and C arereversibly translated relative to each other and layer B provides arestoring force. FIG. 3A demonstrates the force (N) to stretch and causetranslational movement of Layer A relative to Layer C, having a middleelastomeric B layer wherein the hardness of the elastomer impactsdisplacement and restoring force. Example 2 and Table 4 show thelateral/translational restoring force for different B layers.

The term “shearing force,” as used herein means the translational forceapplied to two surfaces which are connected by an elastic material.

The term “shell” is used herein with reference to polymeric shells whichfits over the teeth and are removably placeable over the teeth.

The term “stain resistant” is used herein with reference to a materialdesigned to be resistant to being stained.

The term “thermoplastic polymer” is used herein with reference to apolymer is a polymer that becomes pliable or moldable above a specifictemperature and solidifies upon cooling, provided that the heat andpressure do not chemically decompose the polymer.

The terms “tooth” and “teeth” include natural teeth, including naturalteeth which have been modified by fillings or by crowns, implantedteeth, artificial teeth that are part of a bridge or other fittingsecured to one or more natural or implanted teeth, and artificial teeththat are part of a removable fitting.

In the following description, various embodiments are described. Forpurposes of explanation, specific configurations and details aredescribed in order to provide a thorough understanding of theembodiments. However, it will also be apparent to those skilled in theart that the embodiments may be practiced without the specific details.Furthermore, well-known features may be omitted or simplified in ordernot to obscure the embodiment being described.

EMBODIMENTS

In some embodiments (referred to herein as embodiment #1), athermoformable polymeric sheet, is comprised of at least two outerlayers A and C, and a middle layer B, wherein the A and C layers areindividually comprised of a thermoplastic polymer having a modulus ofgreater than about 1,000 MPa, for example 1,000 MPA to 1,500 MPA; 1,100MPA to 1,600 MPA; 1,200 MPA to 1,700 MPA; 1,300 MPA to 1,800 MPA; 1,400MPA to 1,900 MPA; 1,500 MPA to 2,000 MPA; 1,100 MPA; 1,200 MPA; 1,300MPA; 1,400 MPA; 1,500 MPA; 1,600 MPA; 1,700 MPA; 1,800 MPA, 1,900 MPA;2000 MPA; or up to 2,500 MPA; and a glass transition temperature (Tg)and/or melting point from about 80° C. to 180° C.; 90° C. to 170° C.;100° C. to 160° C.; 110° C. to 150° C.; 120° C. to 150° C.; 130° C. to170° C.; 140° C. to 180° C.; 80° C.; 90° C.; 100° C.; 110° C.; 120° C.;130° C.; 140° C.; 150° C.; 160° C.; 170° C.; or 180° C.

In such embodiments, the middle B layer is comprised of at least anelastomer having a modulus of from about 50 MPa to about 500 MPa; 60 MPato 470 MPa; 70 MPa to 440 MPa; 80 MPa to 400 MPa; 100 MPa to 350 MPa;150 MPa to 300 MPa; 200 MPa to 400 MPa; 60 MPa, 70 MPa; 80 MPa, 90 MPa;100 MPa; 110 MPa; 120 MPa; 130 MPa; 140 MPa; 150 MPa, 160 MPa; 170 MPa;180 MPa; 190 MPa; 200 MPa, 250 MPa, 300 MPa, 350 MPa, 400 MPa, 450 MPa,or up to 500 MPa, and one or more of (a) a glass transition temperature,or (b) a melting point of from about 90° C. to about 220° C.; from 100°C. to about 200° C.; from 120° C. to about 180° C.; from 140° C. to 220°C.; or from 160° C. to about 220° C. In some embodiments, the middle Blayer is an elastomeric layer or shell, which may include one or morematerials and one or more layers.

In embodiment #1, layers A and C may comprise a polyester orco-polyester, a polyurethane, a polyamide, a polyolefin, a (meth)acrylic polymer, a polycarbonate, a vinyl polymer such a polyvinylchloride, or a fluoropolymer.

In embodiment #1, layer B may comprise a polyurethane elastomer, apolyester elastomer, a styrenic elastomer, a polyamide elastomer, asiloxane elastomer, a polyether elastomer a polyolefin elastomer, anolefin copolymer, an acrylic elastomer or a fluroelastomer.

In embodiment #1, the B layer material has a 22 hours at 25° C.compression set of less than about than 35%, 30%, 25%, 20% 10%, lessthan 35%, 34%, 33%, 32%, 31%, 30%, 29%, 28%, 27%, 26%, 25%, 24%, 23%,22%, 21%, 20%, 19%, 18%, 17%, 16%, 15%, 14%, 13%, 12%, 11% or 10%. Incontradiction to the findings of U.S. Pat. No. 9,655,693 B2 where anelastomer is used as an outer layer, we have found that a lowercompression set rather than higher compression set is more effective.

In certain aspects of embodiment #1, the sheet has an overall thicknessof from about 250 microns to about 2,000 microns.

In certain aspects of embodiment #1, the combined thickness of the A andC layers is from about 25 microns to about 1000 microns, 50 microns to750 microns, 100 to 750 microns, 250 microns to 750 microns, or 250microns to about 600 microns.

In certain aspects of embodiment #1, the thermoformable sheet has aflexural modulus of from about 100 MPa to about 2,000 MPa, from about250 MPa to about 2,000 MPa, from about 500 MPa to 1,500 MPa, from about750 MPa to about 2,000 MPa, or from about 750 microns to about 1,500MPa.

In certain aspects of embodiment #1, the A and C layers have a Tg ofbetween about 80° C. and 150° C., and the B layer has a Tg or meltingpoint of between about 180° C. and 220° C. and a heat of fusion of fromabout 5 Joules/g to about 20 Joules/g, or 5 Joules/g to 15 Joules/g.

In certain aspects of embodiment #1, the inter layer peel strength of anA layer is greater than about 50 N/inch, greater than about 60 N/inch,greater than about 70 N/inch.

In certain aspects of embodiment #1, the A and C layers each have athickness of 25 microns to about 1000 microns, 50 microns to 750microns, 100 to 750 microns, 125 to 300 microns, 250 microns to 750microns, or 250 microns to about 600 microns and may have a combinedthickness of about 250 microns to about 600 microns, 200 microns to 300microns, or 150 microns to 250 microns, being comprised of a rigidco-polyester or polyurethane having a modulus of from 1000 MPa to 2,500MPa with Tg of between 95° C. and 150° C., the elastomeric B layerhaving a thickness of from about 200 microns to about 1000 microns or200 to 500 microns e.g., 200 microns, 225 microns, 250 microns, 300microns, 350 microns, 375 microns, 400 microns, 425 microns, 450microns, 475 microns, 500 microns, 550 microns, 600 microns, 650microns, 700 microns, 750 microns, 800 microns, 850 microns, 900microns, 950 microns or 1,000 microns, is comprised of a polyether orpolyester polyurethane having a hardness of from about D 35 to about D65, e.g., D35, D40, D45, D50, D55, D60, or D65, and a 22 hours at 25° C.compression set of less than about than 35%, 30%, 25%, 20% 10%, lessthan 35%, 34%, 33%, 32%, 31%, 30%, 29%, 28%, 27%, 26%, 25%, 24%, 23%,22%, 21%, 20%, 19%, 18%, 17%, 16%, 15%, 14%, 13%, 12%, 11% or 10%, andan A layer that has an inter layer peel strength of greater than about50 N/inch, greater than about 60 N/inch or greater than about 70 N/inch,wherein the polymer sheet has flexural modulus of from about 750 MPa toabout 1,500 MPa; from about 100 MPa to about 2,000 MPa; from about 250MPa to about 2,000 MPa; from about 500 MPa to 1,500 MPa; or from about750 MPa to about 2,000 MPa.

In some aspects of embodiment #1 thin layers of additional polymers (tielayers) may be present to improve the adhesion of polymer layers thatare not naturally adhesive to each other for example a layer of maleicanhydride grafted polypropylene may be used to increase the adhesionbetween a polypropylene A layer and polyester or polyamide B layer.

In some embodiments (referred to herein as embodiment #2), the A and Clayers of the sheet or device can reversibly move relative to each other(for example translationally) from about 0.05 mm to about 0.1 mm with aforce of less than 100 N per cm{circumflex over ( )}2, 50 N percm{circumflex over ( )}2, 25 N per cm{circumflex over ( )}2, or 10 N percm{circumflex over ( )}2.

In some aspects of embodiment #2, the A and C layers of the sheet ordevice have a total thickness of from about 500 microns to 1,000 micronsand can reversibly move relative to each other by a distance 0.05 mm to0.1 mm with a force of less than 100 N per cm{circumflex over ( )}2, 50N per cm{circumflex over ( )}2, 25 N per cm{circumflex over ( )}2, or 10N per cm{circumflex over ( )}2.

In some aspects of embodiment #2, the B layer material has a 22 hours at25° C. compression set of less than about than 35%, 30%, 25%, 20% 10%,less than 35%, 34%, 33%, 32%, 31%, 30%, 29%, 28%, 27%, 26%, 25%, 24%,23%, 22%, 21%, 20%, 19%, 18%, 17%, 16%, 15%, 14%, 13%, 12%, 11% or 10%.

In some embodiments (referred to herein as embodiment #3), one or moreof the A and C layers comprises a microcrystalline polyamide comprisedof from 50 to 100, 50 to 90, 50 to 80, 50 to 70, 60 to 90, 60 to 80, or70 to 90 mole % of C6 to C14 aliphatic diacid moieties, and about 50 to100, 50 to 90, 50 to 80, 50 to 70, 60 to 90, 60 to 80, or 70 to 90 mole% of 4,4′-methylene-bis(cyclohexylamine) (CAS [1761-71-3]), having aglass transition temperature of between about 100° C. and 180° C., aheat of fusion of less than 20 J/g, e.g., 5 Joules/g to about 20Joules/g, or 5 Joules/g to 15 Joules/g. See, e.g., DE Application No. 4310 970 (embodiment 3). In some aspects of embodiments #3, the combinedthickness of the A and C layers is less than about 500 microns, lessthan about 400 microns, less than about 300 microns.

In some embodiments (referred to herein as embodiment #4), a dentalappliance conformal to one or more teeth comprises at least two outerlayers A and C, and a middle layer B wherein the A and C layers, areindividually comprised of a thermoplastic polymer having a modulus ofgreater than about greater than 1,000 MPA, for example 1,000 MPA to1,500 MPA; 1,100 MPA to 1,600 MPA; 1,200 MPA to 1,700 MPA; 1,300 MPA to1,800 MPA; 1,400 MPA to 1,900 MPA; 1,500 MPA to 2,000 MPA; 1,100 MPA;1,200 MPA; 1,300 MPA; 1,400 MPA; 1,500 MPA; 1,600 MPA; 1,700 MPA; 1,800MPA, 1,900 MPA; 2000 MPA; up to 2,500 MPA, in certain aspects greaterthan 1,500 MPa, and a glass transition temperature and/or melting pointfrom about 80° C. to 180° C.; 90° C. to 170° C.; 100° C. to 160° C.;110° C. to 150° C.; 120° C. to 150° C.; 130° C. to 170° C.; 140° C. to180° C.; 80° C.; 90° C.; 100° C.; 110° C.; 120° C.; 130° C.; 140° C.;150° C.; 160° C.; 170° C.: or 180° C., in certain aspects 80 to 150° C.or 95 to 150° C. In such embodiments, the middle B layer is comprised ofat least an elastomer having a modulus of from about 50 MPa to 500 MPa;70 MPa to 450 MPa; 80 MPa to 400 MPa; 100 MPa to 350 MPa; 150 MPa to 300MPa; 200 MPa to 400 MPa; 60 MPa, 70 MPa; 80 MPa, 90 MPa; 100 MPa; 110MPa; 120 MPa; 130 MPa; 140 MPa; 150 MPa, 160 MPa; 170 MPa; 180 MPa; 190MPa; 200 MPa, up to 250 MPa, and one or more of a glass transitiontemperature or melting point of from about 90° C. to about 220° C.

In some aspects of embodiment #4 the A and C layers have a combinedthickness of about 25 microns to about 600 microns, e.g., 100 microns,125 microns, 150 microns, 175 microns, 200 microns, 225 microns, 250microns, 300 microns, 350 microns, 400 microns, 450 microns, 500microns, 550 microns or 600 microns, being comprised of a rigidco-polyester or polyurethane having a modulus of greater than 1,000 MPa,for example 1,000 MPA to 1,500 MPA; 1,100 MPA to 1,600 MPA; 1,200 MPA to1,700 MPA; 1,300 MPA to 1,800 MPA; 1,400 MPA to 1,900 MPA; 1,500 MPA to2,000 MPA; 1,100 MPA; 1,200 MPA; 1,300 MPA; 1,400 MPA; 1,500 MPA; 1,600MPA; 1,700 MPA; 1,800 MPA, 1,900 MPA; 2000 MPA; or up to 2,500 MPA witha Tg of from 80° C. to 180° C.; 90° C. to 170° C.; 100° C. to 160° C.;110° C. to 150° C.; 120° C. to 150° C.; 130° C. to 170° C.; 140° C. to180° C.; 80° C.; 90° C.; 100° C.; 110° C.; 120° C.; 130° C.; 140° C.;150° C.; 160° C.; 170° C.: or 180° C., e.g., 80 to 150° C. or 95 to 150°C.

In some aspects of embodiment #4, the elastomeric B layer has athickness of from about 200 microns to about 1,000 microns, e.g., 100microns, 125 microns, 150 microns, 175 microns, 200 microns, 225microns, 250 microns, 300 microns, 350 microns, 375 microns, 400microns, 425 microns, 450 microns, 475 microns, 500 microns, 550microns, 600 microns, 650 microns, 700 microns, 750 microns, 800microns, 850 microns, 900 microns, 950 microns or 1,000 microns, iscomprised of a polyether or polyester polyurethane having a hardness offrom about D35 to about D65, e.g., D35, D40, D45, D50, D55, D60, or D65,and a 22 hour at 25° C. compression set of less than about 35%, 34%,33%, 32%, 31%, 30%, 29%, 28%, 27%, 26%, 25%, 24%, 23%, 22%, 21%, 20%,19%, 18%, 17%, 16%, 15%, 14%, 13%, 12%, 11% or 10%, and an A layer withan inter layer peel strength of greater than about 50 N/inch, greaterthan about 55 N/inch, greater than about 60 N, greater than about 70 N,wherein the polymeric sheet has flexural modulus of from about 100 MPato about 2,000 MPa, from about 250 MPa to about 2,000 MPa, from about500 MPa to 1,500 MPa, from about 750 MPa to about 2,000 MPa, e.g., fromabout 750 microns to about 1,500 MPa.

In some aspects of embodiment #4, the A and C layers have a lateralrestoring force of 0.05 mm to 0.1 mm with a force of less than 100 N percm², 50 N per cm², 25 N per cm², or 10 N per cm².

In some embodiments (referred to herein as embodiment #5), a dentalappliance is formed by thermoforming a multilayer sheet over a model ofteeth wherein thermoforming is performed at a temperature that is atleast greater than the glass transition temperature and/or melting pointof the outer layers and less than the upper glass transition temperatureand/or melting point of at least an inner layer elastomer material.

In one embodiment of embodiment #5, a dental appliance is prepared bythermoforming a multilayer sheet having least an A and C layer having aTg from about 80° C. to 180° C.; 90° C. to 170° C.; 100° C. to 160° C.;110° C. to 150° C.; 120° C. to 150° C.; 130° C. to 170° C.; 140° C. to180° C.; 80° C.; 90° C.; 100° C.; 110° C.; 120° C.; 130° C.; 140° C.;150° C.; 160° C.; 170° C.; or 180° C., and the B layer has a glasstransition temperature and/or melting point from about 90° C. and 220°C., e.g., 180° C. to 220° C. and a heat of fusion of from about 5 J/g toabout 20 J/g, e.g., about 5 J/g to about 20 Joules/g, or 5 Joules/g to15 Joules/g.

In one aspect of embodiment #5 the A and C layers comprise a copolyesteror polyurethane having a Tg from about 90° C. to about 120° C., the Blayer is comprised of a polyurethane having a modulus of from about 50MPa to 500 MPa and a glass transition temperature and/or melting pointof from about 170° C. to about 220° C. and thermoforming is performed ata temperature between about 150° C. and 200° C.

It should be understood that elements of two or more embodiments may becombined.

In some embodiments, the thermoformable polymeric sheet, is comprised ofat least two outer layers A and C, and a middle layer B, wherein one ormore of the A and C layers comprise a microcrystalline polyamidecomprised of from 50 to 100 mole % of C6 to C14 aliphatic diacidmoieties, and about 50 to 100 mole % of4,4′-methylene-bis(cyclohexylamine) (CAS [1761-71-3]), having a glasstransition of between about 100° C. and 180° C., a heat of fusion ofless than 20 J/g and a light transmission of greater than 80%.

In some embodiments, the thermoformable polymeric sheet, is comprised ofat least two outer layers A and C, and a middle layer B, wherein one ormore of the A and C layers comprises, a co-polyester comprised of adicarboxylic acid component comprising 70 mole % to 100 mole % ofterephthalic acid residues, and a diol component comprising, (i) 0 to95% ethylene glycol, (ii) 5 mole % to 50 mole % of2,2,4,4-tetramethyl-1,3-cyclobutanediol residues, and (ii) 50 mole % to95 mole % 1,4-cyclohexanedimethanol residues, and (iii) 0 to 1% of apolyol having three or more hydroxyl groups, wherein the sum of the mole% of diol residues (i) and (ii) and (iii) amounts to 100 mole % and thecopolyester exhibits a glass transition temperature Tg from 80° C. to150° C. In some aspects of this embodiment, the thermoformable polymericsheet includes a middle B layer which comprises an aromatic polyetherpolyurethane having a Shore hardness of from about A90 to D55 and acompression set of less than 35%, wherein the interlayer peel strengthbetween the A and C layers and the B layer is greater than 50 N per 2.5cm.

In some embodiments, a dental appliance conformal to one or more teethis made from the microcrystalline polyamide or the co-polyesterdescribed above.

In one embodiment, a polymeric sheet composition comprised of at leasttwo rigid or hard outer layers A and C, at least two soft inner layers Band B′ and at least a rigid or hard inner layer, D are provided. The A,C and D layers may be the same or different. The B and B′ layers may bethe same or different.

In some embodiments, the rigid or hard layers, A, C and D, individuallycomprise a thermoplastic polymer having a modulus of from about 1,000MPa to 2,500 MPa.

In some embodiments, the rigid or hard layers, A, C and D individuallycomprise a thermoplastic polymer with a glass transition temperatureand/or melting point of from about 80° C. to 180° C.

In some embodiments, the rigid or hard layers, A, C and D comprise oneor more of a polyester, a co-polyester, a polycarbonate, a polyesterpolycarbonate blend, a polyurethane, a polyamide, a polyolefin, amicrocrystalline polyamide, a co-polyester comprising terephthalic acidand/or isophthalic acid, cyclohexane dimethanol, and2,2,4,4-tetramethyl-1,3-cyclobutanediol, a co-polyester comprisingterephthalic acid and/or isophthalic acid, ethylene glycol anddiethylene glycol, an aromatic polyurethane based on MDI and hexanediol,an aromatic polyurethane with aliphatic diols, polypropylene or aco-polymer of propylene, ethylene and C4 to C8 alpha olefin, acycloaliphatic polyamide, a (meth) acrylic polymer, a vinyl polymer sucha polyvinyl chloride, and a fluoropolymer.

In some embodiments, the inner layers B and B′ individually comprise oneor more of a polyurethane elastomer, an aromatic polyether polyurethane,a polyolefin elastomer, a polyester elastomer, a styrenic elastomer, ananhydride functionalized styrenic elastomer, a polyamide elastomer, apolyether polyamide (polypropylene oxide-based or polytetramethyleneoxide-based), a cyclic olefin elastomer, an acrylic elastomer, anaromatic or aliphatic polyether, a polyester polyurethane, a siloxaneelastomer, a polyether elastomer, an olefin copolymer, an acrylicelastomer and a fluoroelastomer.

In some embodiments, the inner layers B and B′ comprise a polymericmaterial with a hardness of from about A 60 to D 85, from about A 70 toD 75, from about A 80 to D 65, or from about D40 to D70.

In some embodiments, the inner layers B and B′ comprise a polymericmaterial with a glass transition temperature and/or melting point offrom about 90° C. to 220° C.

In some embodiments, the inner layers B and B′ comprise a polymericmaterial with a modulus of from about 50 MPa to 500 MPa.

In some embodiments, the sheet has an overall thickness of from about250 microns to about 2,000 microns, from about 300 microns to about 1900microns, from about 400 microns to about 1750 microns or from about 500microns to about 1500 microns.

In some embodiments, the rigid or hard layers, A, C and D have acombined thickness of from about 100 microns to about 750 microns, fromabout 150 microns to about 600 microns, or from about 200 microns toabout 500 microns.

In some embodiments, the rigid or hard layers, A and C have a combinedthickness of from about 50 microns to about 250 microns, from about 40microns to about 150 microns, or from about 25 microns to about 50microns.

In some embodiments, the soft layers, B and B′ have a combined thicknessof from about 200 microns to about 1,000 microns, from about 250 micronsto about 900 microns, from about 150 microns to about 750 microns.

In some embodiments, a dental appliance conformal to one or more teethis made from a polymeric sheet composition comprising at least layers,A, B, B′, C and D, as described herein. In some embodiments, the orderof these layers may be different.

In some embodiments, the dental appliance has a combined thickness offrom about 250 microns to 2,000 microns and a flexural modulus of fromabout 500 MPa to 1,500 MPa.

In some embodiments, the dental appliance is adapted to sequentiallyposition teeth.

In some embodiments, the dental appliance exhibits improved tearresistance relative to the A, C, or D layer alone.

In some embodiments, the dental appliance exhibits improvedenvironmental stress resistance relative to the A, C or D layer alone.

If one hard outer layer is thinner than another hard outer layer,thermoforming the thinner hard layer against a model can provideimproved contact and conformity to the model increasing comfort, fit,and mechanical force coupling.

In some embodiments, a tooth contacting hard layer on the inside of anappliance has a thickness that is less than another hard layer.

In some embodiments, the thickness of the tooth contacting hard layercan be less than about 250 microns or as thin as about 50 microns.

In one embodiment, a polymeric sheet composition is comprised of threeor more layers wherein one outermost hard layer (A1) has a differentthickness, than another outermost hard layer (A). In some embodiments,one outermost hard layer (A1) is thinner than another outermost hardlayer (A).

In some embodiments, outermost hard layer (A1) has a thickness of fromabout 25 to about 250 microns, or from about 25 to about 150 microns, orfrom about 25 to about 100 microns.

In some embodiments, the ratio of the thickness of an outermost hardlayer (A1) to a thickness of a second outermost hard layer (A) is lessthan about 0.9, 0.85, 0.8, 0.75, 0.7, 0.6, 0.5, 0.35, 0.25, 0.15, e.g.,from about 0.9 to about 0.2, from about 0.8 to about 0.3, from about 0.5to about 0.15.

In some embodiments, the thickness of a polymeric sheet compositioncomprising an outermost hard layer (A), an inner soft layer B, and anoutermost hard layer (A1) is from about 500 microns to about 2,000microns, or from about 625 microns to about 1,000 microns. In someaspects of this embodiment, the ratio of the thickness of an outermosthard layer (A1) to a thickness of a second outermost hard layer (A) isfrom about from about 0.9 to about 0.2, from about 0.8 to about 0.3, orfrom about 0.75 to about 0.25.

TABLE 9 Exemplary Thickness of Layers. Thickness (microns) Ratio Total AB A1 A1:A Symmetric 1 750 250 250 250 1.00 Symmetric 2 624 187 250 1871.00 Non-symmetric 1 750 250 375 125 0.50 Non-symmetric 2 750 375 250125 0.33 Non-symmetric 3 750 400 250 100 0.25 Non-symmetric 4 750 425250 75 0.18 Non-symmetric 5 875 350 425 100 0.29 Non-symmetric 6 1000375 525 100 0.27

In some embodiments, the flexural modulus of a polymeric sheetcomposition comprising an outermost hard layer (A), inner soft layer B,and outermost hard layer (A1) is from about 100 MPa to about 2,000 MPa,from about 250 MPa to about 2,000 MPa, from about 500 MPa to 1,500 MPa,from about 750 MPa to about 2,000 MPa, or from about 750 microns toabout 1,500 MPa.

In some embodiments, the flexural modulus of an outermost hard layer (A)and outermost hard layer (A1) is from about 1000 MPa to 2,500 MPa, fromabout 1,000 MPA to about 1,500 MPA; from about 1,100 MPA to about 1,600MPA; from about 1,200 MPA to about 1,700 MPA; from about 1,300 MPA toabout 1,800 MPA; from about 1,400 MPA to about 1,900 MPA; from about1,500 MPA to about 2,000 MPA; about 1,100 MPA; about 1,200 MPA; about1,300 MPA; about 1,400 MPA; about 1,500 MPA; about 1,600 MPA; about1,700 MPA; about 1,800 MPA, about 1,900 MPA; about 2000 MPA; or up to2,500 MPA. The flexural modulus of the outermost hard layer (A) andoutermost hard layer (A1) may be the same or different.

In some embodiments, the Shore hardness of an inner soft layer B, isfrom about A 60 to about D 85, from about A 65 to about D 80, from aboutA 70 to about D 75, from about A 85 to about D 70, or from about A 90 toabout D 65; or from about D 35 to about D 65.

In some embodiments, the glass transition temperature or melting pointof an outermost hard layer (A) and outermost hard layer (A1) is fromabout 80° C. to 180° C.; 90° C. to 170° C.; 100° C. to 160° C.; 110° C.to 150° C.; 120° C. to 150° C.; 130° C. to 170° C.; 140° C. to 180° C.;80° C.; 90° C.; 100° C.; 110° C.; 120° C.; 130° C.; 140° C.; 150° C.;160° C.; 170° C.; or 180° C.; 95° C. to 150° C.

In some embodiments, the glass transition temperature or melting pointof an inner soft layer B is from about 90° C. to about 220° C.; from100° C. to about 200° C.; from 120° C. to about 180° C.; from 140° C. to220° C.; or from 160° C. to about 220° C.

In some embodiments, the compression set of an inner soft layer B isless than about 35%, 30%, 25%, 20% or 10%, less than 35%, 34%, 33%, 32%,31%, 30%, 29%, 28%, 27%, 26%, 25%, 24%, 23%, 22%, 21%, 20%, 19%, 18%,17%, 16%, 15%, 14%, 13%, 12%, 11% or 10% after 22 hours at 25° C.

In some embodiments, outermost hard layer (A) and outermost hard layer(A1) are comprised of one of more of a co-polyester, a polycarbonate, apolyester polycarbonate blend, a polyurethane, a polyamide or apolyolefin. In some embodiments, the outermost hard layer (A) andoutermost hard layer (A1) comprise the same material. In someembodiments, the outermost hard layer (A) and outermost hard layer (A1)comprise different materials.

In some embodiments, a middle B layer is comprised of one or more of apolyurethane elastomer, a polyolefin elastomer, a polyester elastomer, astyrenic elastomer, a polyamide elastomer, a cyclic olefin elastomer, anacrylic elastomer, an aromatic or aliphatic polyether and a polyesterpolyurethane.

Construction Methods

Multilayer sheets may be prepared by a number of means including withoutlimitation, hot or cold lamination, adhesive lamination, meltlamination, coextrusion multilayer extrusion or other known methods.Sheets may be fully prepared before forming into an orthodonticappliance, or an appliance may be produced using a sequence ofindividual thermoforming steps to create multiple layers.

Thermoforming of sheets to produce test samples or dental appliances maybe performed using a “Biostar” pressure former available from GreatLakes Orthodontics using procedures commonly used in the industry.Alternatively, thermoforming may be performed using a roll fedthermoformer, a vacuum former or other known thermoforming techniques.Thermoforming may be conducted using different conditions, forms ormodels to vary draw ratio and part thickness. Multilayer appliances maybe fabricated through one or more 3D printing processes or by sequentialdip coating, spray coating, powder coating or similar processes knownfor producing films, sheets and 3D structures.

Sheet temperature during thermoforming can be measured using an infraredthermometer or a surface thermocouple.

Utility

The sheets and materials described herein have utility as thermoformablematerials having superior dimensional stability, impact cushioning, andrestorative forces. The sheets may be converted into a number of typesof oral appliances, for example for moving teeth, for use as a sportsmouth guard with improved impact resistance and for use as anorthodontic retainer. Improved properties of the materials andappliances described herein relative to currently available materialsand appliances include but are not limited to greater flexibilityresulting in improved end user comfort, improved tooth movement results,greater stain and stress cracking resistance and excellent cosmetics allof which promote more consistent wear by the subjects.

Test Methods

Tensile properties were measured using an Instron Universal MaterialsTester.

The procedures of ASTM D638 were employed unless otherwise noted. Colorand transparency were measured using a BYK Gardner Spin colorimeter.

Impact resistance was measured using a Gardner impact tester. Tearstrength was measure using an Materials Tester at a rate of 250 mm perminute.

Stress relaxation of samples at 37° C. in water were measured by themethod described in U.S. Pat. No. 8,716,425 B2.

Resistance to staining was measured by exposing test articles to astaining medium such as mustard or coffee for 24 hours at 37° C. andmeasuring color on a white color tile before and after exposure.

Translational recovery force was measured by constructing a three-layerstructure (or polymer sheet) as shown in FIGS. 1 and 2. Samples weredisplaced from 0 to 0.5 mm and the force reported in N/cm{circumflexover ( )}2.

Inter layer peel strength is measured at a rate of 50 mm/min and may bereported as Newtons (N) per inch or per 2.54 cm (N). Details may befound in test method ASTM D3164.

Thermal testing to determine glass transition temperatures, melting, andfreezing points were measured using a differential scanning calorimeterat a heating and cooling rate of 10° C. per minute unless indicatedotherwise.

Resistance to environmental stress cracking may be determined byfixturing a sheet sample around a cylindrical mandrel to induce aspecified strain on the outer surface, for example 3% or 5%, andexposing the samples to a specified environment for a specified time,for example a saliva mimic solution, mouth rinse or other solution ofinterest. The response can be measured semi quantitatively by visualobservation of type and number of cracks, or quantitively bysubsequently measuring a mechanical property such as tear strength.

Materials and Methods.

Materials of construction. A large number of commercially availablematerials can be utilized in producing the sheets and appliancesdescribed herein. Table 1 provides a listing of exemplary materials foruse in the A or C component. Table 2 provides a listing of exemplarymaterials for use in the B component. Similar or related materials canbe obtained from other manufacturers or produced by known methods.

TABLE 1 Exemplary Materials Useful as Primary Components of A or CMaterials Modulus Hardness Trade Name Supplier Chemical Composition Tgor Tm Range Range Tritan MX Eastman Co-polyester of Tg 100 to 1,000 to R100 710, MX 810, Chemical terephthalic acid, 120° C. 1,500 MPa to 115 MP100 MP cyclohexane dimethanol, 200 and 2,2,4,4-tetramethyl-1,3-cyclobutanediol. Eastar 6763 Eastman Co-polyester of Tg 80° C. 2,000to R106 Chemical terephthalic acid, 2,100 MPa ethylene glycol anddiethylene glycol Isoplast 2530 Lubrizol Aromatic polyurethane Tg 85-95°C. 1,900 MPa R 121 based on MDI and hexanediol Isoplast 2531 LubrizolAromatic polyurethane Tg 95-110° C. 2,100 MPa R 121 with aliphatic diolsPolypropylene Generic Co-polymer of propylene, Tm 135 to 1,000 to D55-65 co-polymer ethylene and C4 to C8 160° C. 1,500 MPa alpha olefinTrogamide Evonik Cycloaliphatic polyamide Tg 140° C. 1,400 MPa D 81CX7323 Tm 250° C.

TABLE 2 Exemplary Materials Useful as Primary Components of B MaterialsCompression Chemical Tg or set 22 Hr. Modulus * Hardness Trade NameSupplier Composition Tm° C. @ 23° C. MPa Range Elastollan BASF Aromaticpolyether Tm 180-200 30% 52 95 A 1195A polyurethane Texin RxT CovestroAromatic polyether Tm 180-200 20% 151 50 D 50D polyurethane ElastollanBASF Aromatic polyether Tm 180-200 14% 57 85 A 1185 A polyurethane Texin985 Covestro Aromatic polyether Tm180-200 17% 60 85 A polyurethane PebaxClear 300 Arkema Polyether polyamide TM 160 <20%  183 53 D Kraton FG1901 Kraton Maleated SEBS Tg 90-100 15% 25 71 A Polymers Noito MitsuiPropylene - ethylene Tm Est120 C. 20% 30 75 A Chemical microcrystallineelastomer Vestamide Evonik Polytetramethlene Tm Est. 170 25 to 40% 50 to500 D40 to D70 E or ME ether polyamide to 220 C. block polymer * Modulusfrom vendor literature or estimated based on Shore hardness

Additional suitable materials for the A, B or C layers can includecompatible or incompatible blends, for example blends of two or moreco-polyesters, blends of polypropylene and polyethylene and ethylenepropylene elastomers, fluoropolymers, such as polyvinylidene fluoride orits co-polymers, styrene acrylonitrile resins, acrylonitrile styrenebutadiene resins (ABS), polyurethanes containing polycarbonate softblocks, siloxane soft blocks, silicone elastomers such as Geniomer™, asiloxane urea co-polymer, and cyclic olefin co-polymers and cyclicolefin elastomers.

EXAMPLES

The disclosure is further illustrated by the following examples. Theexamples are provided for illustrative purposes only. They are not to beconstrued as limiting the scope or content of the disclosure in any way.

Example 1

A series of monolayer and multilayer sheets of nominal total thickness0.76 mm were prepared as shown in Table 3. Test samples 1-4 wereprepared by compression molding and heat laminating individual films orby extrusion lamination. Examples of prior art materials, P1, P2 and P3,were prepared by compression molding films and optionally heatlaminating them.

Press lamination was conducted at 200 to 220° C., extrusion laminationwas done using a polyurethane melt temperature of 210 to 240° C., andcoextrusion was done with a polyester melt temperature of 240° C. to260° C., and a polyurethane melt temperature of 210 to 240° C.Conditions of time, temperature and pressure were varied to maximizestructure (polymer sheet) quality, thickness and adhesion.

Mechanical properties, optical properties, stress relaxation and shaperecovery were measured to compare the suitability of resultingstructures (polymer sheets).

TABLE 3 Monolayer and Multilayer Sheets Material Property P1 P2 P3 1 2 34 Construction Monolayer Monolayer ABA ABC ABC ABC ABC P1 layer/micronsIsoplast Polyester Texin Polyester Polyester Isoplast Polyester 2530 A250 950 75 B 250 B250 2531 B 175 P2 layer/microns Isoplast PolyesterPolyester Elastollan Texin Texin Texin 2530 A 250 A 600 1185A RxT 50DRxT 950 LW 65D P3 layer/microns Isoplast Polyester Texin PolyesterPolyester Isoplast Polyester 2530 A 250 950 75 B 250 B 250 2531 B 175Flexural Modulus 1910 Mpa 1750 Mpa 675 Mpa 824 Mpa 968 Mpa 729 Mpa 575Mpa Elongation to 100-130 100-130 100-130 100-130 100-130 100-130100-130 break (%) Impact Strength 15.2 6.9 — — 9.2 14 — J/mm TrouserTear 18 48 51 156 165 187 170 Force (N) Load retention g(1) 0 days 42443956 2664 2638 2739 2567 1598 1 day 707 853 875 1329 1469 1534 1232 2days 498 601 823 1257 1389 1398 1247 Color change 0.97 2.1 44 2.6 2.31.1 2.2 dB(2) * Polyester A is a copolyester marketed by EastmanChemical (Eastar 6763). Polyester B is cycloaliphatic copolyestermarketed by Eastman Chemical under the tradename Tritan (1)5% strain/37°C./water (2)Mustard/24 hr/22° C.

Prior art material P1 is a commercial thermoformable aligner materialsupplied by Bay Materials, LLC, Fremont Ca. Prior art material P2 is apolyester having a glass transition temperature of about 90° C.manufactured by Eastman Chemical sold under the trade name Eastar 6763.Prior art material P3 is described in U.S. Pat. No. 9,655,693 B2. Testsamples 1-4 are multilayer laminates (as described herein),demonstrating improved stress relaxation properties, increased tearstrength and excellent stain resistance.

Compared to the prior art materials, test samples 1-4 exhibited a numberof unexpected properties. Comparing test samples 1-4 and prior artmaterials P1 and P2, it can be seen that test samples 1-4 exhibitsubstantially lower initial forces in the stress relaxation test(believed to translate to greater user comfort), but surprisingly,maintain the forces for a longer time. This is in contradiction to theteachings of U.S. Pat. No. 9,655,693 B2, which teaches that an outerlayer of elastomer is required to protect the inner hard layer. Theability of multilayer sheets to maintain appropriate force levels forlong time periods under demanding conditions can readily be seen in FIG.4. Samples A and B in FIG. 4 are monolayer sheets while samples 1 and 2are multilayer sheets as described in Table 3.

Tear strength is an important property of dental appliances. Materialswith low tear strength have low durability and may crack at locationswhere stress is concentrated. Comparing the tear strength of prior artmaterials P1, P2 and P3, with test samples 1-4 show that such multilayerstructures (or polymer sheets) with an elastomeric B layer havesignificantly greater tear strength than comparable monolayer structuresor prior art multilayer structured.

To further investigate the effect of construction on tear strength,another laminate (#5) was prepared with 0.25 mm A and C layers comprisedof Eastar 6763, a copolyester available from Eastman Chemical having aTg of 86° C. and a 0.2 mm B layer of Shore 50 D urethane elastomer togive a total thickness of 0.7 mm. Tear strength of this sample wascompared to prior art materials P1, P2 and P3. Sample #5 exhibited atear strength of 120 N, more than 200% of the value of prior artmaterial P3, while having similar ratios of polyurethane and polyester.

Example 2 (Measurement of Translational Force)

A three-layer sheet was prepared as described in Example 1 for testmaterial 2. A strip of the sheet 2.54 cm×1 cm was bonded between twostrips of rigid polyester 2.54 cm wide to create 0.5 cm overlap(“multilayer sample A2”). A control test sample was prepared using thesame size and thickness of polyester A (prior art) between two strips ofrigid polyester. The displacement/force response was measured at a rateof 0.04 MPa/min and results are reported in Table 4. The multilayerconstruction allows the two outer layers (or the two shells) of anappliance to accommodate greater elastic movement with appropriateforces than the prior art constructions.

TABLE 4 Controlled Elastic Movement of Multilayer Material DisplacementForce (N)/cm² (mm) Polyester A 95 A 50 D 0 0 0 0 0.1 43 8 12 0.25 106 2031 0.5 249 50 71

Orthodontic devices were made using the materials and methods describedherein and compared to devices of the same shape and thickness made fromZendura A and Essix Plus. The disclosed devices were substantially moreelastic and more comfortable to wear. Because the inner and outer shellscan deform independently from each other, they can accommodate a greateroffset between the actual teeth and the appliance without causing unduediscomfort to the patient and exert a near constant force for long timeperiods to accurately move teeth.

Example 3

A clarified polypropylene film designated BFI 257 supplied by Blue RidgeFilms (Petersburg, Va.), with a thickness of 0.25 mm was laminated toboth sides of 0.25 mm thick film prepared from Kraton GF (maleated SEBS,available from Kraton Polymers) in a hot press at 180 F, cooled and cutinto a 125 mm circle. The modulus of the polypropylene is reported at1,100 MPa. The SEBS elastomer has a reported hardness of 71 A and amodulus of 25 MPa. The multilayer film exhibited low staining and wasthermoformable over a dental model to produce a retainer with excellentelastic recovery properties.

Example 4

The durability of sheet materials in the presence mouthwash wasinvestigated since it is known that dental appliances may be readilydamaged by alcohols and/or surfactants. Test sheets with a thickness of0.75 mm were prepared 2.54 cm wide×12 cm length. Prior art materials P1,P2 and P3, and multilayer sheet (test material) #2 were wrapped on amandrel of sufficient diameter to produce a strain of 5%. The sampleswere immersed in mouth rinse and maintained at 37° C. This environmentis known to promote environmental stress cracking and to induce setcausing the materials to be in a hoop shape instead of flat. After 24hours, the samples were rinsed with deionized water and the amount ofrecovery was measured immediately and again after 24 and 48 hours atambient temperature. Subsequently, the samples were viewed under amicroscope to determine the amount of stress cracking on the side whichwas under extension. A sample which returned to completely flat isscored to have 100% recovery. Stress cracking was rated from 1 to 5,with 5 being no visible cracking and 1 being severe cracking. Shaperecovery for the samples is given in Table 5. The multilayer sheet (#2)recovered more rapidly and more completely than prior art materials P1,P2 and P3.

TABLE 5 Shape Recovery of Samples % Recovery Minutes P1 P2 P3 #2 0.01 5334 51.6 66 60 54 49 53.8 69 1440 61 65 59.4 77

Example 5

Three laminates were prepared as in Example 1, sample 2 and designatedas samples #6, #7 and #8. Sample #6 was extrusion laminated usinguntreated polyester film at a roll temperature of 40° C., sample #7 wasextrusion laminated using corona treated polyester film at a rolltemperature of 60° C. and sample #8 was extrusion laminated using coronatreated film at a roll temperature of 80° C. Corona treatment iscommonly used to activate film surfaces to increase their polarity. Acontrol sample of polyester A was designated sample #9. The mechanicalproperties and environmental stress cracking resistance of the threesamples are given in Table 6.

TABLE 6 Effect of Inter Layer Peel Strength on ESC Resistance of A LayerProperty #6 #7 #8 #9 Modulus (Mpa) 1,490 1,572 1,589 2,700 Elongation atYield (%) 6.1 5.8 6.1 6.2 Elongation at Break (%) 124 131 129 131 InterLayer Peel Strength 35 53 137 NA (N/inch) Tear Strength 55 117 179 48ESCR/Mouthrinse/37° C. 1 2.5 4 1

The dramatic improvement in environmental resistance observed forsamples #7 and #8 compared to samples #6 and #9 are unanticipated andunexpected. In each case the material exposed to the environment ischemically identical and under equal amounts of stress. While notwishing to be bound by theory we hypothesize that some concentratedstrain induced stress present in the outer polyester layer may betransferred to the elastomeric material and the force transfer is moreefficient in the materials having higher interlayer bond strength.However, we are not aware of any precedent for this result.

It is well known that thermoplastic non-crystalline co-polyesters (PETGsand PCTGs) have poor environmental stress cracking resistance and areprone to rapid degradation when used as dental appliances. U.S. Pat. No.9,655,691 teaches that covering both sides of such a co-polyester with athermoplastic polyurethane elastomer having a hardness of from about 60A to about 85 D surprisingly increased the durability of dental alignersmade from such materials (described as a “hard polymer layer disposedbetween two soft polymer layers”). Presumably the outer materialprovides a physical and/or chemical protective layer. A disadvantage ofsuch materials is that the polyurethane elastomers and other elastomershave poor staining resistance, and the disclosed multilayer structurehas poor tear resistance.

The inventors have unexpectedly discovered that the stress crackingresistance of amorphous polyester films, sheets, or thermoformed partsprepared therefrom can be dramatically improved by bonding anelastomeric material such as a polyurethane between two layers of thepolyester. The resulting structure, having a soft polymer layer disposedbetween two hard polymer layers, has excellent chemical resistance, hightransparency, and excellent stain resistance. Additionally, the tearresistance of the multilayer structure is greater than either thepolyester or the elastomer alone. The inventors have also discoveredthat the improved properties require a high bond strength between thelayers and that a material with poorly bonded layers has inferiorcracking resistance and inferior tear strength.

It is known in the art that rigid polyurethane sheets by themselves havevery good stress cracking resistance. Unexpectedly, we observed that athree layer ABA structure which has rigid polyurethane A (outer) layersand an elastomeric B (inner) layer with excellent adhesion had WORSEenvironmental stress cracking resistance than the rigid polyurethanealone, the opposite effect to that observed with a polyester outerlayer.

Example 6

Testing was conducted to investigate the effect of thermal treatment andthermoforming conditions on the performance of devices made from thesheets. Three sheets (2A, 2B and 2C) of test material 2 (three-layer,polyester, polyurethane, polyester), were dried at 60° C. under vacuumfor 12 hours. The samples were put in moisture barrier bags andsubjected to the thermal treatment and thermoforming conditions shown inTable 7. Sample 2A was maintained at 22° C. and samples 2B and 2C wereannealed at 100° C. for 24 hours. The samples were then thermoformed toproduce flat sheet using different thermoforming temperatures. Samples2A and 2B where thermoformed at a temperature below the high end of themelting range of the polyurethane while 2C was thermoformed at atemperature above the melting range of the polyurethane.

TABLE 7 Effect of Thermal Treatment/Thermoforming Conditions on RetainedStress Sample Treatment and Performance Sample/Treatment #2A #2B #2CTemperature 22° C. 100° C. 100° C. Time 24 hrs. 24 hrs. 24 hrs. Tm (°C.) 160-190 170-195 170-195 J/g 6.8 12.4 12.6 Thermoforming 180 180 200temperature Tm 160-190 C. 160-210 160-200 J/g 6.3 8.3 6.4 RetainedStress 51% 72% 47% 24 hr.

Test samples were cut from thermoformed samples, analyzed by DSC andsubjected to stress relaxation testing at 37° C. in water. DSC showedthat the melting point and heat of fusion of the samples were increasedby annealing at 100° C. and that the thermoforming reduced the amount ofheat of fusion and melting range. However, the sample thermoformed belowthe upper melting range of the polyurethane retained more crystallinityand performed better in the stress relaxation test. The conditions forsample 2B in Table 7 were used to fabricate a dental appliance.

Example 7

Additional compositions can be made by selection of suitable layermaterials having differences in modulus and elasticity as shown in Table8.

TABLE 8 Exemplary Multilayer Sheet Materials Sample #6 Sample #7 Sample#8 Layers Material Thickness Material Thickness Material Thickness ATrogamide CX 0.125 mm Trogamide CX 0.2 mm Altuglas 0.125 mm 73237323/Polyamide 614 Luctor CR 13 A′ Blend 90:10 Altuglas SG10 0.125 mm BPebax Clear 300 0.5 mm Pellethane 0.3 mm Kurarity 0.25 mm B′ 2373 55DLA4285 C′ Trogamide 0.125 mm Trogamide CX 0.2 mm Altuglas SG10 0.125 mmC CX 7323 7323/Polyamide 614 Altuglas Luctor 0.125 mm Blend 90:10 CR 13Altuglas SG10 is a transparent impact modified polymethyl methacrylatesold be Arkema Altuglas Luctor CR13 is a transparent impact modifiedpolymethyl methacrylate sold be Arkema Kurarity LA4285 is an acrylic ABAblock co-polymer of methyl methacrylate and butyl methacrylate

Example 8

A 2 mm thick sheet was prepared by laminating two outer films ofpolypropylene homopolymer 0.250 mm thick (Blue Ridge Films BFI 3270,modulus 1,200 MPa) and an inner layer of 1.50 mm thick ethylenepropylene microcrystalline elastomer (Noito PN 2070, Mitsui Chemical)modulus 150 MPa. The sheet was cut into a disc 125 mm in diameter andthermoformed over a model of the maxillary teeth of an individual andtrimmed to make a highly impact resistant sports mouth guard.Surprisingly, the mouth guard provides better impact protection andcomfort than a standard device fabricated from 4 mm thick ethylene vinylacetate copolymer marketed by Dreve under the Tradename Drufosoft.

Example 9

An aligner was made by thermoforming a three-layer sheet over a model ofteeth. Two outer layers were comprised of a rigid polyurethane having aTg of about 120° C. and an inner B layer comprised of a Shore A 85aromatic polyether polyurethane having a hard block melting point of 160to 195° C. and heat of fusion of 8 J/gram. The appliance was annealed at100° C. for 24 hours which is below the Tg of the outer layer. Nodeformation was observed. Testing demonstrated that this appliance wasmore elastic and exhibited less creep under load than the beforeannealing at 100° C. The improvement is thought to be due to improvementin the microstructure of the polyurethane elastomer.

In a second test a comparison was made between a multilayer device and asingle layer device where in each case Zendura A materials was used asthe A/C material or as the A/B/C material respectively. The devices wereannealed at 90° C. for 24 hours. It was observed that the monolayerdevice extensively deformed while the multilayer maintained its shape.It is hypothesized that in the multilayer device the elastomer maintainsa stabilizing force on the more rigid material during annealing toprevent undesired dimensional changes.

What is claimed is:
 1. A polymeric sheet composition, comprising: atleast two outer layers A and C and an elastomeric inner layer B, whereinone or both of the outer layers A and C individually comprise athermoplastic polymer having a flexural modulus of from about 1,000 MPato 2,500 Mpa, and the inner layer B is comprised of an elastomericmaterial having a hardness from about A 60 to D
 85. 2. The polymericsheet composition of claim 1, wherein the A, B and C layers of thepolymeric sheet composition have a combined thickness of from 250microns to 2,000 microns and a flexural modulus of from 500 MPa to 1,500MPa
 3. The polymeric sheet composition of claim 2, wherein the outerlayers A and C comprise one or more of a polyester, a co-polyester, apolycarbonate, a polyester polycarbonate blend, a polyurethane, apolyamide, a polyolefin, a microcrystalline polyamide, a co-polyester ofterephthalic acid, cyclohexane dimethanol, and2,2,4,4-tetramethyl-1,3-cyclobutanediol, a co-polyester of terephthalicacid, ethylene glycol and diethylene glycol, an aromatic polyurethanebased on MDI and hexanediol, an aromatic polyurethane with aliphaticdiols, a co-polymer of propylene, ethylene and C4 to C8 alpha olefin, acycloaliphatic polyamide, a (meth) acrylic polymer, a vinyl polymer sucha polyvinyl chloride, and a fluoropolymer.
 4. The polymeric sheetcomposition of claim 2, wherein the inner layer B comprises one or moreof a polyurethane elastomer, an aromatic polyether polyurethane, apolyolefin elastomer, a polyester elastomer, a styrenic elastomer, apolyamide elastomer, a polyether polyamide (polypropylene oxide-based orpolytetramethlene oxide-based), a cyclic olefin elastomer, an acrylicelastomer, an aromatic or aliphatic polyether, a polyester polyurethane,a siloxane elastomer, a polyether elastomer, a polyolefin elastomer, anolefin copolymer, and a fluoroelastomer.
 5. The polymeric sheetcomposition of claim 3, wherein one or more of outer layers A and Ccomprise a microcrystalline polyamide having heat of fusion of less than20 J/g and a light transmission of greater than 80%.
 6. The polymericsheet composition of claim 4, wherein the inner layer B comprises ananhydride functionalized styrenic elastomer having a hardness of fromabout A60 to D50.
 7. The polymeric sheet composition of claim 2, whereinone or more of the A and C layers comprises a co-polyester having a: (a)a dicarboxylic acid component and (b) a diol component, whereinthe-copolyester exhibits a glass transition temperature (Tg) from 80° C.to 150° C.
 8. The polymeric sheet composition of claim 2, wherein one ormore of the A and C layers comprises a polyurethane comprised of: (a) adi-isocyanate, and (b) a diol component comprising one or more of hexanediol and cyclohexane dimethanol wherein the polyurethane has a glasstransition temperature Tg from about 85° C. to about 150° C.
 9. Thepolymeric sheet composition of claim 2, wherein the inner layer Bcomprises an aromatic polyether polyurethane having a compression set ofless than 35%.
 10. The polymeric sheet composition according to claim 2,wherein the inner layer B comprises an aromatic polyether polyurethanehaving an interlayer peel strength between the A and C layers and the Blayer of greater than 50 N per 2.5 cm.
 11. A polymeric sheet compositioncomprised of at least two rigid or hard outer layers A and C, at leasttwo soft inner layers B and B′, and at least a rigid or hard innerlayer, D, wherein the rigid or hard layers, A, C and D, individuallycomprise a thermoplastic polymer having a modulus of from about 1,000MPa to 2,500 MPa and the soft inner layers B and B′ individuallycomprise an elastomeric material having a hardness from about A 60 to D85.
 12. The polymeric sheet composition of claim 11, wherein thepolymeric sheet composition has a combined thickness of from 250 micronsto 2,000 microns and a flexural modulus of from 500 MPa to 1,500 MPa.13. The polymeric sheet composition of claim 12, wherein the rigid orhard layers, A, C and D individually comprise one or more of apolyester, a co-polyester, a polycarbonate, a polyester polycarbonateblend, a polyurethane, a polyamide, a polyolefin, a microcrystallinepolyamide, a co-polyester of terephthalic and/or isophthalic acid,cyclohexane dimethanol, and 2,2,4,4-tetramethyl-1,3-cyclobutanediol, aco-polyester of terephthalic and/or isophthalic acid, ethylene glycoland diethylene glycol, an aromatic polyurethane based on MDI andhexanediol, an aromatic polyurethane with aliphatic diols, a co-polymerof propylene, ethylene and C4 to C8 alpha olefin, a cycloaliphaticpolyamide, a (meth) acrylic polymer, a vinyl polymer such a polyvinylchloride, and a fluoropolymer.
 14. The polymeric sheet composition ofclaim 12, wherein the soft inner layers B and B′ individually compriseone of more of a polyurethane elastomer, an aromatic polyetherpolyurethane, a polyolefin elastomer, a polyester elastomer, a styrenicelastomer, an anhydride functionalized styrenic elastomer, a polyamideelastomer, a polyether polyamide (polypropylene oxide-based orpolytetramethylene oxide-based), a cyclic olefin elastomer, an acrylicelastomer, an aromatic or aliphatic polyether, a polyester polyurethane,a siloxane elastomer, a polyether elastomer, a polyolefin elastomer, anolefin copolymer, an acrylic elastomer and a fluoroelastomer.
 15. Thepolymeric sheet composition of claim 14, wherein one or more of the softinner layers B and B′ comprise an anhydride functionalized styrenicelastomer having a hardness of from about A60 to D50.
 16. The polymericsheet composition of claim 14, wherein the rigid or hard layers, A, Cand D have a combined thickness of from about 100 microns to about 750microns.
 17. The polymeric sheet composition of claim 14, wherein thesoft layers, B and B′ have a combined thickness of from about 150microns to about 1,000 microns.
 18. A dental appliance conformal to oneor more teeth comprising a polymeric sheet composition according toclaim
 12. 19. A polymeric sheet composition, comprising: three or morelayers, A, A1, and B, wherein the outermost layers, A and A1 havedifferent thicknesses from one another and individually comprise athermoplastic polymer having a flexural modulus of from about 1,000 MPato 2,500 MPa, and wherein the inner layer B is comprised of anelastomeric material having a hardness from about A 60 to D
 85. 20. Thepolymeric sheet composition of claim 19, wherein the A, A1 and B layersof the polymeric sheet composition have a combined thickness of fromabout 500 microns to 2,000 microns.
 21. The polymeric sheet compositionof claim 20, wherein the outermost layer A1 has a thickness of fromabout 25 to about 250 microns.
 22. The polymeric sheet composition ofclaim 20, wherein the ratio of the thickness of the outermost layer A1to the thickness of the outermost layer A is from about 0.9 to about0.2.
 23. The polymeric sheet composition of claim 20, wherein theoutermost layer A and outermost layer A1 are comprised of one or more ofa polyester, a co-polyester, a polycarbonate, a polyester polycarbonateblend, a polyurethane, a polyamide and a polyolefin.
 24. The dentalappliance according to claim 20, wherein the outer layers A and A1comprise the same material.
 25. The dental appliance according to claim20, wherein the outer layers A and A2 comprise different materials. 26.The polymeric sheet composition of claim 20, wherein the middle layer Bis comprised of one or more of a polyurethane elastomer, a polyolefinelastomer, a polyester elastomer, a styrenic elastomer, a polyamideelastomer, a cyclic olefin elastomer, an acrylic elastomer, an aromaticor aliphatic polyether and a polyester polyurethane.
 27. A dentalappliance conformal to one or more teeth comprising a polymeric sheetcomposition according to claim
 20. 28. The dental appliance of claim 27,wherein the dental appliance exhibits improved tear resistance relativeto the A, C or D layer alone.
 29. The dental appliance of claim 27,wherein the dental appliance exhibits improved environmental stressresistance relative to the A, C or D layer alone.